1. Field of the Invention
The present invention relates to a glass antenna system for an automobile, which is most suitable for use in a diversity antenna system wherein a plurality of antennas are switchable optionally from one to another.
2. Description of the Prior Art
A glass antenna comprising antenna strips formed on or in a window glass of an automobile has become widely used as an antenna for a radio receiver in an automobile. In such a glass antenna, there has been an improvement of the pattern of the antenna strips or an improvement of an amplifier, a choke coil, a capacitor, etc. which are associated with the antenna, and there are available some glass antennas having superior non-directivity or sensitivity. For instance, a glass antenna 6 as shown in FIG. 1 is used for an automobile as an antenna having superior sensitivity to a FM broadcast wave since it has minimum horizontal directivity when it receives a horizontally polarized wave of the FM broadcast band, wherein there are formed on the surface of a glass plate 5, an asymmetric antenna pattern 1, a feed point 2 at one side of the glass plate and a defogging electric heating element provided with a pair of bus bars, one of which is divided into upper and lower bus bars 3 and 3' at one side.
However, even such a glass antenna has a drawback that it often exhibits sharp directivity when it receives a radio wave transmitted from a transmitting antenna of a radio broadcast station, under certain conditions. For instance, when it receives the radio wave at a place surrounded by tail buildings or at a place where an influence of a reflected wave is great, its non-directivity and sensitivity tends to be degraded. Namely, a ultra-short wave like a FM broadcast wave propagates linearly, and at a place surrounded by tall buildings or mountains, the radio wave transmitted from the radio station will be received by the receiving antenna, not directly but after reflected by such obstacles as the tall buildings or mountains. The plane of polarization of the radio wave propagated via such a complicated route tends to be distorted even when the plane of polarization of the transmitted wave is horizontal, i.e., the plane of polarization tends to incline toward a V-component (i.e., a vertical component).
The above mentioned antenna is designed primarily for a horizontally polarized wave which is most effective for improvement of the non-directivity for the FM broadcast wave. Accordingly, it tends to exhibit directivity with respect to the actual radio wave containing various polarized wave components, in the city or at a place surrounded by mountains, and the sensitivity is thereby reduced, for instance, in a case where the horizontally polarized wave component is weak and other polarized wave component such as a vertical wave component is strong, namely at a place where the ratio of the polarized wave components (i.e. H/V ratio, Hi 40 dB.mu./m, Vi 30 dB.mu./m) is 10 dB.mu./m.
To improve the function for receiving a radio wave containing various polarized wave components, it is known to use a diversity antenna system wherein a plurality of antennas having different directivities are provided so that it is possible to selectively use one of the antennas which has better sensitivity and directivity against the particular radio wave received and containing various polarized wave components.